/// 测试字符串的引用
pub fn test_str_ref() {
    let (s1, s2) = ("some", "thing");

    let s = String::from(s1) + s2;
    println!("{}", s);

    let mut s = String::from(s1);
    s.push_str(s2);
    println!("{}", s);

    let s = format!("{}{}", s1, s2);
    println!("{}", s);

    let s = [s1, s2].concat();
    println!("{}", s);
}
